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RASA 8 Halos?

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#1 Maryland Mike

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Posted 24 January 2020 - 06:38 PM

I’m noticing a lot of “halos” if that’s the right word on RASA 8 images with bright stars, an example attached. Is this caused by overexposing the image, or is this something specific to the RASA or something else?

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#2 Jim Waters

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Posted 24 January 2020 - 07:47 PM

Is this a cropped image?  Are you sure you are focused, correct spacing for camera...?  What camera are you using?


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#3 Maryland Mike

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Posted 24 January 2020 - 09:41 PM

Not my image, yes it is cropped.  I pulled it from AstroBin. 



#4 Jim Waters

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Posted 24 January 2020 - 09:54 PM

If its not your image it would be best to provide a link to the image you have questions about.



#5 clivemilne

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Posted 26 January 2020 - 06:41 PM

Mike, it is intrinsic to any optical system that has a flat(ish) refractive element in the entrance pupil of the telescope. (Full aperture Schmidt plates satisfy this condition)

What's happening is that the light not absorbed by the CCD is reflected back towards the primary mirror which then collimates the beam... By that I mean the primary makes the light parallel again or focused to infinity.

When this light hits the corrector plate, 1% of it (give or take) is reflected back towards the primary mirror which focuses it back in to a slightly defocused "ghost" image on the CCD.

This is the Achilles heal of Schmidt based systems.

The only practical solution is to design the Schmidt plate to include a zero power meniscus in to its form. There was an optical company in Europe (Lichtenknecker) which offered flat field Schmidts with curved corrector plates...  To my understanding no one else has?


Edited by clivemilne, 26 January 2020 - 10:05 PM.

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#6 Maryland Mike

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Posted 26 January 2020 - 09:32 PM

Thanks for that detailed explanation, Clive. I’ve been considering a RASA 8 and noticed this feature on bright stars while perusing AstroBin and was curious what exactly I was seeing. Now I know.


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